1. Field of the Invention
The present invention relates to a semiconductor device and to a method of fabrication thereof, and more particularly to a method of fabricating a semiconductor device having a metal silicide film on a conductor or a diffused layer.
2. Description of the Related Art
To improve the degree of integration of LSI, it is necessary to reduce the depth of the junction of the impurities diffused layer in the source/drain regions. A reduced thickness of the diffused layer, however, increases the resistance value of the diffused layer and results in a reduced operating speed of the semiconductor. In view of this, semiconductor devices have conventionally been fabricated by the method described below.
First, device isolation regions are formed on a P-type silicon substrate, and a plurality of gate electrodes, each including a gate oxide film and a polycrystal silicon, are sequentially formed on the substrate. A small amount of N-type impurities are injected into the substrate, and a silicon oxide film is formed over the entire surface of the substrate. This silicon oxide film is anisotropically etched, thereby forming sidewall oxide films on the side walls of each gate electrode. Further, ion implantation of N-type impurities into the substrate with high energy forms a MOSFET of LDD (lightly doped drain) structure. After that, a metal film is formed over the entire surface of the substrate, and a metal silicide film is formed on each of the impurity diffused layers and the gate electrodes by heat treatment. The portion of the metal film that has not been converted into a silicide is then removed. These steps fabricate a MOSFET with the source/drain resistance reduced by the metal silicide film.
In carrying out ion implantation into the silicon substrate, a silicon oxide film is formed as a protective film on the substrate surface to prevent the substrate surface from roughening and the implant profile defect due to the channeling, and ions are implanted through this silicon oxide film. Ion implantation through an oxide film, however, results in introduction of oxygen into the silicon substrate due to the bombardment of the implanted ions and develops a phenomenon called "knock-on". The proportion of oxygen thus knocked on increases when impurities large in mass such as arsenic are implanted. When a metal silicide layer is formed on the impurity diffused layer formed by ion implantation, the knock-on phenomenon causes the oxygen introduced into the silicon substrate to suppress the formation of a silicide. As a result the thickness of the metal silicide layer is reduced to such an extent that it is difficult to reduce the resistance of the source/drain regions. This problem is especially conspicuous with the N-type impurity diffused layer subjected to ion-implantation of arsenic.
In manufacture of a P-type MOSFET by the above-mentioned fabrication method, boron may be implanted as impurities into a polysilicon film constituting a gate electrode. Since the diffusion coefficient of boron is high, however, boron in the gate electrode is diffused into the channel region of the substrate through the gate oxide film thereby causing variations in the threshold voltage of the MOSFET.
JP-A-64-760 discloses a fabrication method in which oxygen ions are not introduced into the silicon substrate and the silicon substrate surface is thus prevented from becoming rough. More specifically, after sidewall oxide films are formed, a silicon nitride film is formed over the entire surface of the substrate, and impurities are implanted into the substrate with high energy through the silicon nitride film, thereby forming an impurity diffused layer. In this method, nitrogen, not oxygen, is introduced into the silicon substrate and the gate electrodes by knock-on, and therefore conversion into silicide is not suppressed.
This method, however, requires the additional steps of forming a silicon nitride film over the entire surface of a silicon substrate after forming sidewall oxide films on the side walls of each gate electrode by anisotropic etching of a silicon oxide film, and also removal of the silicon nitride film used as a protective film by using hot phosphoric acid after implanting impurities with high energy. Thus, the fabrication steps of a semiconductor device become complicated. The same semiconductor device is fabricated in large quantities at a time, and therefore, a change in even a single fabrication step has a great effect on the production cost and poses a very serious problem.